Document handling of Node.js versions, OS, and arch. (#7072)
### Description
While we may consider OS and arch handling as a feature in the future,
we don't today. Here, we're documenting how to handle this advanced use
case as well as jotting down a proper way of handling Node.js versions.
Closes https://github.com/vercel/turbo/issues/4124 and
https://github.com/vercel/turbo/issues/1315
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Co-authored-by: Nicholas Yang <nicholas.yang@vercel.com>
